Our overall goal for this research plan is to use a mouse model system for pneumonic plague to discover and evaluate Y. pestis genes critical for the development and progression of disease. We will pinpoint these candidates using two methods: transcriptional profiling to reveal genes that are differentially regulated in the various stages of pneumonic plague;and forward genetics approaches to screen/select for Y. pestis genes that are indispensable for development of pulmonary disease. Specific Aim 1. Comparative transcriptional responses by Y. pestis during the stages of pneumonic plague. We previously developed a whole genome microarray to characterize the bacterial transcriptome during pneumonic plague, but this analysis was technically limited to a late stage of infection. Therefore, we will use quantitative RT-PCR to examine a subset of Y. peso's genes throughout the entire time course of disease. This subset of approximately 250 genes is based on genes that show evidence of differential expression during infection, as well as genes that were not sufficiently explored by microarray technology. Specific Aim 2. Forward genetics to identify bacterial genes important in the development of pneumonic plague. Transposon site hybridization (TraSH) is a gene discovery strategy using negative selection to dentify bacterial genes that are essential during infection. The microarrays we have constructed will allow us to take advantage of a a TraSH-based approach using array hybridizations to identify Y. pestis genes triplicated in various stages of the pulmonary infection. Specific Aim 3. Analyzing the importance and role of candidate virulence-associated genes. The genes selected in the first two Aims will be targeted for further analysis by creating defined mutant strains of Y. pestis. Mutant and control strains will be tested for virulence in the murine model of pneumonic plague, monitoring bacterial proliferation in the lung, dissemination to the spleen, and histopathology to evaluate differences in the manifestation or kinetics of disease. The characterization of mutant strains will be extended to a microarray analysis of host transcriptional responses during infection, done in collaboration with Dr. Virginia Miller.